Elastomeric lubricants for magnetic recording media

ABSTRACT

Elastomeric fluoropolyether lubricant topcoats are formed on a protective overcoat of a magnetic recording medium. Embodiments include forming a an amorphous carbon layer having a thickness of no greater than about 25 Å as the protective overcoat and polymerizing a fluorpolyether having one or more acrylate groups on the amorphous carbon layer to form the elastomeric fluoropolyether topcoat lubricant directly on the protective overcoat.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/559,096filed Apr. 27, 2000, now U.S. Pat. No. 6,617,011, which claims priorityfrom Provisional Application Ser. No. 60/133,216, filed May 7, 1999.

FIELD OF THE INVENTION

The present invention relates to magnetic recording media, such as thinfilm magnetic disks, having a polymerized lubricant topcoat thereon. Thepresent invention has particular applicability in manufacturing magneticrecording media suitable for high density recording and long termmagnetic performance stability.

BACKGROUND OF THE INVENTION

Thin film magnetic recording disks and disk drives are conventionallyemployed for storing large amounts of data in magnetizable form. Inoperation, a typical contact start/stop (CSS) method commences when adata transducing head begins to slide against the surface of the disk asthe disk begins to rotate. Upon reaching a predetermined high rotationalspeed, the head floats in air at a predetermined distance from thesurface of the disk where it is maintained during reading and recordingoperations. Upon terminating operation of the disk drive, the head againbegins to slide against the surface of the disk and eventually stops incontact with and pressing against the disk. Each time the head and diskassembly is driven, the sliding surface of the head repeats the cyclicoperation consisting of stopping, sliding against the surface of thedisk, floating in the air, sliding against the surface of the disk andstopping.

For optimum consistency and predictability, it is necessary to maintaineach transducer head as close to its associated recording surface aspossible, i.e., to minimize the flying height of the head. Accordingly,a smooth recording surface is preferred, as well as a smooth opposingsurface of the associated transducer head. However, if the head surfaceand the recording surface are too flat, the precision match of thesesurfaces gives rise to excessive stiction and friction during the startup, flying and stopping phases, thereby causing wear to the head andrecording surfaces, eventually leading to what is referred to as a “headcrash.” Thus, there are competing goals of reduced head/disk frictionand minimum transducer flying height.

Conventional practices for addressing these apparent competingobjectives involve providing a magnetic disk with a roughened recordingsurface to reduce the head/disk friction by techniques generallyreferred to as “texturing.” Conventional texturing techniques includelaser texturing the surface of a non-magnetic substrate to provide atextured landing zone in which a magnetic head can land when the driveis not in use, and can take off when the drive is reading and writingdata. Typically, the surface of the non-magnetic substrate is polishedto a specular finish prior to a laser texturing process to form alanding zone leaving a substantially smooth data zone. Subsequently, anunderlayer, a magnetic layer, a protective overcoat and a lubricanttopcoat are sequentially deposited, wherein the textured surface on thesubstrate is intended to be substantially replicated in the subsequentlydeposited layers. Typical substrate materials include an aluminum alloywith a layer of amorphous nickel phosphorous thereon, glasses, ceramicsand glass-ceramic materials, as well as graphite. Underlayers typicallycomprise chromium or a chromium alloy, while the magnetic layertypically comprises a cobalt based alloy. Protective overcoats typicallycontain carbon. Such layers are typically deposited by sputteringtechniques preformed in an apparatus containing sequential depositionchambers.

The application of the protective layer and the topical lubricant areessential for high durability and reliability of thin film recordingmedia. In accordance with conventional practices, a lubricant topcoat isapplied over the protective layer to prevent wear between the disk andhead interface during drive operation.

Fluoropolyether lubricants are of particular interest in lubricatingmagnetic recording media. These lubricants are uniquely suited to formlubricant topcoats on magnetic media because of such properties aschemical inertness, low vapor pressure, low surface tension, highthermal stability, stability under high shear stress and good boundarylubrication properties. Among the many lubricants available, liquidperfluoropolyethers (PFPE) are the most typically used in formingtopcoat lubricants on magnetic recording media.

Liquid lubrication of the disk surface encounters several problems,however, which limit its effectiveness as used in rotating storagemedia. For example, it is well known that non-bonded lubricants willspin off a thin film disk with a carbon overcoat. Typically, PFPElubricants do not have a retention means so that when the disk rotates,the lubricant tends to spins off the disk. The depletion of thelubricant from the disk surface increases the friction between the diskand the read/write head.

Further, the depletion of the lubricant results in non-uniformity acrossthe surface of the disk resulting in additional operationaldifficulties. For example, where the thickness is too thin, the head cancause wear on the disk surface and where the lubricant thickness is toogreat, the head will become stuck in the lubricant (from staticfriction) and the head or disk could be damaged when the head suddenlybecomes unstuck due to the rotating disk.

Burguette et al. in U.S. Pat. No. 4,404,247 disclose anchoring apolymerizable composition directly to a metallized substrate by acomplex system which includes an inner polymeric film and an outerpolymeric film. The inner polymer is made from a film forming aromaticor heterocyclic polymerizable monomer and a vinyl aromatic polymer andthe outer polymer contains a compound having a perfluoropolyethersegment. Burguette et al disclose that such a system would adequatelyprotect a metallic thin film and teach away from the use of a hardprotective coating on magnetic thin film media. Several other patents toBurguette et al., such as U.S. Pat. Nos. 4,526,833; 4,569,962;4,671,999; and 4,705,699, disclose additional ingredients in creatingthe complex two phase polymer coating system.

Accordingly, a continuing need exists in the art for an improvedlubricated magnetic recording medium. In particular there exists a needfor an efficient, cost-effective method of manufacturing a magneticrecording medium with a lubricant topcoat exhibiting improvedtribological performance and fly-stiction.

SUMMARY OF THE INVENTION

An advantage of the present invention is an efficient cost-effectivemethod of manufacturing a magnetic recording medium having a lubricanttopcoat optimized for tribological performance with reducedfly-stiction.

An additional advantage of the present invention is a magnetic recordingmedium having an elastomeric lubricant topcoat thereon with improvedtribological performance.

Additional advantages and other features of the present invention willbe set forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from the practice of thepresent invention. The advantages of the present invention may berealized and obtained as particularly pointed out in the appendedclaims.

According to the present invention, the foregoing and other aspects areachieved in part by a method of manufacturing a magnetic recordingmedium with an elastomeric topcoat lubricant. The method comprisesforming a protective overcoat on a magnetic layer; applying afluoropolyether having at least one polymerizable group thereon to theprotective overcoat; and polymerizing the fluoropolyether to form anelastomeric topcoat lubricant directly on the protective overcoat.

In embodiments of the present invention, the fluoropolyether, e.g. aperfluoropolyalkylether having more than one acrylate groups, can bepolymerized to a crosslinked elastomeric fluoropolyether directly on theprotective overcoat with improved tribology. In accordance withembodiments of the present invention, a protective overcoat, e.g. acarbon containing overcoat, can be formed to a thickness of no greaterthan about 50 Å, e.g. no greater than about 25 Å, with the elastomericfluoropolyether lubricant topcoat directly on the protective overcoat.The elastomeric topcoat of the present invention advantageously permitsa thinner protective overcoat thereby minimizing the fly height of thetransducer head over the magnetic recording medium consistent with thedemands for higher areal recording density.

Another aspect of the present invention is a magnetic recording mediumcomprising a protective overcoat and an elastomeric fluoropolyethertopcoat directly on the protective overcoat. In an embodiment of thepresent invention, the magnetic recording medium comprises a substrate;a magnetic layer on the substrate; the protective overcoat on themagnetic layer, e.g. an amorphous carbon layer having a thickness of nogreater than about 25 Å; and the elastomeric fluoropolyether topcoatlubricant directly on the protective overcoat.

The inventive magnetic recording medium advantageously comprises aprotective overcoat and an elastomeric fluoropolyether topcoat withimproved tribological performance at a combined thickness of no greaterthan about 60 Å. The advantageously thin protective overcoat andlubricant topcoat minimizes the distance between the magnetic layer andthe transducer head improving signal and recording performance of theinventive magnetic media when incorporated in a magnetic recordingdevice.

Additional advantages of the present invention will become readilyapparent to those having ordinary skill in the art from the followingdetailed description, wherein the embodiments of the invention aredescribed, simply by way of illustration of the best mode contemplatedfor carrying out the invention. As will be realized, the invention iscapable of other and different embodiments, and its several details arecapable of modifications in various obvious respects, all withoutdeparting from the invention. Accordingly, the drawings and descriptionare to be regarded as illustrative in nature, and not as restrictive.

DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically illustrates comparative results of CSS testing of aconventionally lubricated magnetic recording disk to a magneticrecording disk having an elastomeric lubricant topcoat according to thepresent invention.

FIG. 2 shows CSS durability results for a magnetic recording mediumhaving an elastomeric lubricant topcoat according to the presentinvention.

FIG. 3 shows the stiction superiority of a magnetic recording diskhaving an elastomeric lubricant topcoat according to the presentinvention.

DESCRIPTION OF THE INVENTION

Surprisingly, it was found, through experimentation and investigation,that the combined thickness of an elastomeric topcoat lubricant on aprotective overcoat of a magnetic recording medium can be minimized toan optimal thickness with excellent lubricity and wear resistance. Thereduced thickness of the lubricating and protective layer minimizes thedistance between the transducer head and the magnetic layer of arecording device thereby improving signal and device performance. Thepresent invention provides an improved, efficient and cost effectivemethod of optimizing tribological performance of a magnetic recordingmedium.

In accordance with the present invention, a lubricant topcoat is formedon a protective overcoat of a magnetic recording medium. The magneticrecording medium of the present invention comprises a composite of amagnetic layer on a non-magnetic substrate. In an embodiment of thepresent invention, an underlayer such as chromium or a chromium alloy isformed between the non-magnetic substrate and the magnetic layer.

In practicing the present invention, the protective overcoat comprisesone or more layers of carbon, e.g. hydrogenated amorphous carbon ornitrogenated amorphous carbon. Hydrogenated amorphous carbon, forexample, is hydrogen doped amorphous carbon, while nitrogenatedamorphous carbon is nitrogen doped amorphous carbon. Other thin filmmaterials that serve as a protective overcoat in accordance withembodiments of the present invention include silicon nitride.Embodiments of the present invention also includes dual or multiplelayers serving as the protective overcoat as, for example, a bilayer ofsilicon nitride capped with hydrogenated carbon. The protective overcoatis formed on the magnetic medium by conventional techniques as, forexample, by sputter-depositing an amorphous hydrogenated carbon filmemploying a plasma deposition chamber. A more complete description offorming a protective overcoat can be found, for example, in U.S. Pat.No. 5,945,191 to Hwang et al.

Typical non-magnetic substrate materials include an aluminum alloy witha layer of amorphous nickel-phosphorous thereon, such as analuminum-magnesium alloy with a nickel-phosphorous surface coating.Other substrate materials include glasses, ceramics and glass-ceramicmaterials, as well as graphite. A magnetic recording layer of thepresent invention includes typical magnetic recording layers as, forexample, a cobalt based alloy layer, such as one or more layerscomprising one or more of chromium, platinum, tantalum, niobium,tungsten with cobalt. Other magnetic layer materials contemplated by thepresent invention include a barium ferrite film and perpendicularrecording media. The magnetic layer or layers and underlayer aredeposited on the non-magnetic substrate by conventional depositiontechniques, as by sputtering techniques preformed in an apparatuscontaining sequential deposition chambers.

In accordance with the present invention, the protective overcoat andthe elastomeric lubricant topcoat are formed on the magnetic layer ofthe magnetic recording medium to a minimum thickness. Advantageously,the protective overcoat and elastomeric lubricant topcoat of the presentinvention can be formed as thin as necessary for protecting andlubricating the magnetic medium, as for example, a protective overcoatof less than about 10 Å. Embodiments include forming the protectiveovercoat to a thickness of no greater than approximately 25 Å, andforming the elastomeric lubricant topcoat directly on the protectiveovercoat to a thickness of approximately 5 Å to about 30 Å.

The protective and lubricant layers of the present invention providelubricity and wear resistance at a minimum thickness thereby reducingthe distance between the magnetic layer and the transducer head of arecording device. In an embodiment of the present invention, theprotective overcoat and the elastomeric lubricant topcoat aresequentially formed over the magnetic layer having a combined thicknessof no greater than about 60 Å, e.g. a combined thickness of no greaterthan about 30 Å.

In practicing the present invention, an elastomeric lubricant topcoat isformed on the protective overcoat by applying a fluoropolyether havingat least one polymerizable group thereon to the protective overcoat andpolymerizing the fluoropolyether to form an elastomeric topcoatlubricant directly on the protective overcoat. Fluoropolyetherlubricants of the present invention include compounds having theformula:A-Rf-B

wherein Rf is a fluoropolyether; and A is an alkyl group, alkoxy group,a polymerizable group, or a fluorinated derivative thereof, and B is apolymerizable group. In an embodiment of the present invention, both Aand B represent polymerizable groups, e.g., both A and B are acrylates.Polymerization of such a polyfunctional fluoropolyether results in acrosslinked elastomeric lubricant topcoat on the protective overcoat.

Polymerizable groups according to the present invention include thosemoieties that can repeatedly undergo chemical unions with themselves as,for example, vinyl groups, such as alkenes, alkynes, acrylates,acrylamides, epoxy groups, carboxyl groups, etc. Polymerizable groups ofthe present invention also comprise those moieties that chemically joinwith a complementary moiety as, for example, carboxylic acids andalcohols, carboxylic acids and amines, anhydrides and amines,isocyanates and amines, alcohols and/or acids, etc. Accordingly, thepresent invention contemplates addition polymerization, condensationpolymerization and/or ring-opening polymerization process inpolymerizing the applied fluoropolyether lubricant to form theelastomeric topcoat lubricant on the magnetic media.

Fluoropolyethers of the present invention include homopolymers, randompolymers or block polymers, i.e. the repeating units of thefluoropolyether, Rf, may be the same or different. In addition,different repeat units of Rf can be randomly distributed along thebackbone of the polymer or distributed as a block of one type of repeatunit and subsequent blocks of different repeat units along the backboneof the polymer. The inventive lubricants can be completely fluorinatedor partially fluorinated and can be linear or branched. In an embodimentof the present invention, Rf is a perfluoropolyalkylether comprising aplurality of —(C_(a)F_(2a)O)_(n)— repeating units, wherein subscript ais independently in each such unit an integer of from 1 to about 10 andn is an integer from 2 to about 100. In an embodiment of the presentinvention n is an integer from about 5 to about 80, e.g. from about 10to about 60.

Fluoropolyether lubricants having at least one polymerizable group arecommercially available or can be prepared by conventional syntheticchemistry. Representative fluoropolyalkylethers of the foregoing havethe structures shown in the Table

TABLE¹ 1

2

3 F(CF₂CF₂CF₂O)_(n)CX₂CX₂OC(O)CH═CH₂ 4F(CF₂CF₂CF₂O)_(n)CX₂CX₂NHC(O)CH═CH₂ 5CF₃O(CF₂CF(CF₃)O)_(n)CX₂CX₂OC(O)CH═CH₂ ¹wherein each of n and m are 1 toabout 200 and X is hydrogen or fluorine.

The fluoropolyalkylethers derivatives of the above are commerciallyavailable and can be prepared as described in the U.S. Pat. Nos.4,681,925 and 3,814,741.

In an embodiment of the present invention, the fluoropolyetherlubricants have a number average molecular weight of about 500 to about20000 a.m.u., e.g., about 1000 to about 2500 a.m.u. In practicing thepresent invention, the fluorpolyether lubricants can be applied to aprotective overcoat of a magnetic recording medium in any convenientmanner, as by dip coating the medium in a solution of the lubricant in aconventional organic solvent or by applying a solution of the lubricantover the medium. Conventional spray, wipe and vapor coating techniquesfor applying the lubricants are also contemplated by the presentinvention.

In an embodiment of the present invention, the lubricant is dissolved ina conventional solvent, such as Freon, Vertrel XF or perfluorohexane(solvents available from Dupont), in a ratio of about 0.0001% to about100% by (weight/weight), e.g. about 0.001% to about 1%. A typicalmagnetic recording medium, for example, a composite comprising anon-magnetic substrate having sequentially deposited on each sidethereof an underlayer, a magnetic layer, and a protective carbonovercoat, is submerged in the lubricant solution and then slowlywithdrawn therefrom. In practicing the present invention, one can employa conventional lifter-type dipper to submerge the composite in thelubricant solution. One having ordinary skill in the art can easilyoptimize the duration of submergence and the speed of withdrawal toachieve a desired coating thickness.

After application of the fluoropolyether lubricant having at least onepolymerizable group, the fluoropolyether is then polymerized and/orcrosslinked in accordance with the type of polymerizable moiety on thefluoropolyether. For example, polymerization can be carried out by theapplication of heat for condensation polymerizable groups, or by the useof conventional radiation, a catalyst system or heat for additionpolymerizable groups. The resulting polymers are elastomeric and exhibitflexibility and excellent wear resistance as elastomeric lubricanttopcoats.

In an embodiment of the present invention, a fluoropolyethers having atleast one vinyl moiety is advantageously polymerized directly on thesurface of a protective overcoat of a magnetic recording medium. Thefluoropolyether can be polymerized according to photochemical methods,such as ultraviolet (UV) radiation, or by bombardment with anhigh-energy electron beam. For example, UV radiation can be used toeffect polymerization of the fluoropolyether lubricant in an inertatmosphere with or without the use of a photoinitiator. Although therate of polymerization is enhanced by photoinitiators or sensitizers,adequate polymerization of the lubricant can be attained without theiruse.

Further, ultraviolet radiation is particularly effective and economicalfor crosslinking fluoropolyether lubricants with more than one vinylgroups, e.g., multiple acrylate groups. It is possible to vary themolecular weights of the starting fluoropolyether and the number ofvinyl groups to regulate the degree of crosslinking when afluoropolyether having multiple polymerizable groups is employed to formthe elastomeric topcoat lubricant.

In an embodiment of the present invention, the lubricated magneticrecording medium is exposed to UV radiation from a low pressure mercurylamp UV lamp which emits radiation over the range of about 254 nm toabout 185 nm. In an embodiment of the present invention, the lubricateddisk is exposed to UV radiation having an intensity of about several toabout 100 or more milliwatts/cm² for an exposure duration ofapproximately 5 seconds to about 10 minutes to form a polymerizedelastomeric fluorpolyether lubricant topcoat.

EXAMPLES

Perfluoroalkylpolyether diacrylate lubricants having the formula:

wherein m is about 1 to about 50; n is about 1 to about 50 and Xrepresents either hydrogen or fluorine were obtained from 3M (located inSt. Paul, Minn.). The diacrylate lubricant were dissolved in PF5060solvent (available from 3M) at about 0.01% to about 1% by weight to formsolutions of the lubricant in the solvent. The lubricant solutions wereapplied to amorphous carbon protective layers of magnetic recordingmedia by using a dip-coat technique, i.e. by dipping the media in thesolution and slowly withdrawing the media from the solution to yield alubricated amorphous carbon layer.

In the present experiment, thin film magnetic media having an amorphouscarbon protective overcoat were used. The carbon protective overcoatswere formed by conventionally sputtering techniques performed in anapparatus (available from Intevac of Santa Clara, Calif.) containingsequential deposition chambers, to form the amorphous carbon overcoatshaving a thickness of about 10 Å to about 40 Å on the magnetic layer ofrigid disks. The lubricant solutions were then applied directly to theamorphous carbon overcoats.

The thickness of the lubricant on the media was controlled by varyingthe rate of withdrawal and/or solution concentration to yieldthicknesses in the range of about 10 Å to about 30 Å. The thicknesseswere measured by FTIR (Fourier Transform Infrared Spectroscopy),calibrated by x-ray reflectometry and ESCA (Electronic Spectroscopy forChemical Analysis), which has an error on the order of about 1 Å.

The applied diacrylate lubricants were then polymerized to a crosslinkedelastomeric lubricant topcoat directly on the amorphous carbon overcoatto form a combined thickness of the lubricant layer together with thecarbon layer of about 10 Å to about 40 Å. The polymerization conditionswere as follows: the lubricated disks were exposed to a UV lamp with anintensity of about 5 milliwatts/cm² in a UV chamber for about 90seconds.

The superior tribological performance of magnetic recording mediaprepared by the present invention compared to conventionally preparedmagnetic recording media is shown in FIGS. 1-3. In the comparison, arigid magnetic recording disk having a 93 Å bumped height landing zonewas provided. In accordance with the present example, an approximately25 Å thick amorphous carbon protective overcoat was deposited on amagnetic layer and the crosslinked elastomeric topcoat was formed at athickness of about 26 Å on the amorphous carbon layer. The conventionalmagnetic recording disk was prepared by depositing a 50 Å amorphouscarbon layer onto a magnetic layer and then depositing a 23 Å thickconventional high molecular weight Z-DOL lubricant on the amorphouscarbon layer.

FIG. 1 illustrates the comparative initial stiction for the two disks.As shown, by the line identified with the symbol (a), the conventionallyprepared disks typically have an observed stiction value of greater than2.5 grams. In contrast, the disks prepared by the present invention havea stiction value (shown as line (b)) of no greater than approximately1.5 grams, e.g., no greater than about 1.25 grams, even though thethickness of the lubricant layer and carbon layer for the inventive diskare less than a conventionally prepared disk.

The superior tribological performance is further evident by the highdurability observed for the disks prepared by the present invention. Asthe results demonstrate in FIG. 2, the inventive disks have beenprepared having a stiction value that does not exceed about 1.5 gramsfor over 30,000 Contact-Start-Stop cycles. FIG. 3 graphically displaysthe superior parking stiction of less than about 1.75 grams of theinventive disk (s-Lube) compared to the conventional disk (l-Lube). Theobserved results for the magnetic recording media prepared according tothe present invention demonstrate superior tribology while minimizingthe thickness of the lubricant topcoat and protective overcoat.

The present invention is not limited to any particular type of magneticrecording medium, but can be employed in any of various magneticrecording media, including those wherein the substrate or a subsequentlydeposited base layer has been textured, as by mechanical treatment or bylaser techniques, and the textured surface substantially reproduced onsubsequently deposited layers. Thus, a lubricant prepared in accordancewith the present invention, can be applied to form an elastomericlubricant topcoat, such as a crosslinked elastomeric topcoat directly onthe surface of a protective overcoat of a magnetic recording medium, butis not necessarily limited thereto.

Only the preferred embodiment of the present invention and an example ofits versatility is shown and described in the present disclosure. It isto be understood that the present invention is capable of use in variousother combinations and environments and is capable of changes ormodifications within the scope of the inventive concept as expressedherein.

1. A method of manufacturing a magnetic recording medium, the methodcomprising: forming a amorphous carbon protective overcoat having athickness of no greater than about 25 Å over a magnetic recording layer;applying a fluoropolyether having at least one polymerizable groupthereon to the protective overcoat; and polymerizing the fluoropolyetherto form an elastomeric topcoat lubricant directly on the protectiveovercoat, wherein said protective overcoat and said elastomeric topcoatlubricant have a combined thickness of no greater than 60 Å.
 2. Themethod according to claim 1, comprising polymerizing the fluoropolyetherto a crosslinked elastomeric fluoropolyether directly on the protectiveovercoat.
 3. The method according to claim 1, comprising applying thefluoropolyether to a thickness of about 10 Å to about 30 Å.
 4. Themethod according to claim 1, wherein lubricant is aperfluoropolyalkylether having one or more acrylate groups.
 5. Themethod according to claim 4, wherein the fluoropolyether comprises aplurality of —(C_(a)F_(2a)O)_(n)— repeating units, wherein subscript ais independently in each such unit an integer of from 1 to about 10 andn is an integer from 5 to about
 80. 6. The method according to claim 1,wherein the fluoropolyether comprises a compound having the formula:CH₂═CH(O)COCH₂CF₂O(CF₂CF₂O)_(n)(CF₂O)_(m)CF₂CF₂OC(O)CH═CH₂ wherein eachof n and m is 1 to about
 200. 7. The method according to claim 1,comprising polymerizing the fluoropolyether by exposure to UV radiation.8. The method according to claim 1, wherein said elastomeric topcoatlubricant comprises the polymerization product of a fluoropolyetherhaving at least two polymerizable acyrlate groups through whichpolymerization occurs.